1. Field of the Invention
The present invention relates to radio transceivers, and more particularly to combinations of receiving antennas and transmitting antennas that reduce signal losses that occur before the first low-noise amplification or after the last power amplification stage.
2. Description of Related Art
Radio transceivers that share one antenna between the receiver and transmitter generally rely on a transmit/receive (T/R) switch to multiplex the antenna. This, of course presupposes half-duplex operation where only the receiver or transmitter is being used at any one time.
Many radio receiver applications are such that they must reliably receive and demodulate very weak signals. The radio field signal strength can drop so low in some applications that special low-noise amplifier (LNA) modules are integrated with the antenna assembly. A typical such application is the microwave patch antennas used in global positioning system (GPS) navigation receivers. Of course, GPS receivers do not have the complication of an included transmitter or T/R switch. Similarly, losses between the power amplification and the antennas due to diversity and T/R switches limits the available transmitted power and impacts on the transmission reliability.
Antenna diversity is used in some modern systems to take advantage of the fact that even closely positioned antennas can experience very different induced signal levels. This often requires a “diversity switch” that can be controlled to select one antenna over the other.
In radio transceiver systems that operate in the 5.0+ GHz spectrum, e.g., radio transceivers that conform to the IEEE 802.11 standard, a typical T/R switch and diversity switch can each insert approximately 1.5 dB of signal loss. At the field strengths that wireless local area networks operate in, their receivers can ill-afford such losses because they come before the earliest point an LNA can be placed. Another approximately 2.0 dB of loss is typically introduced when relatively complex bandpass filters, e.g., bandpass filters with 3 or more poles are added between the antenna diversity switch and T/R switch to eliminate spurious sidebands from the transmitted signal. Such a filter sits in the received signal path too. Altogether, such losses in conventional designs can add up to approximately 5.0 dB for each of the transmitter and receiver to the antenna.
Local area networks (LAN's) are conventionally interconnected by twisted-wire pairs and shielded cables. A whole class of mobile computing has emerged that depends on various kinds of wireless communication. Amateur radios and cellphones have long been used to connect computer browsers to the Internet, and now wireless devices have been used to build LAN's within a room or building. Better frequency bands and higher allowable power will allow LAN clients to roam around in cities and along highways.
An industry-standard, the IEEE 802.11 specification, defines the protocol for two types of networks, ad-hoc and client/server networks. An ad-hoc network is a simple network where communications are established between multiple stations in a given coverage area without the use of an access point or server. The standard specifies the etiquette that each station must observe so that they all have fair access to the wireless media. It provides methods for arbitrating requests to use the media to ensure that throughput is maximized for all of the users in the base service set. The client/server network uses an access point that controls the allocation of transmit time for all stations and allows mobile stations to roam from cell to cell. The access point is used to handle traffic from the mobile radio to the wired or wireless backbone of the client/server network. This arrangement allows for point coordination of all of the stations in the basic service area and ensures proper handling of the data traffic. The access point routes data between the stations and other wireless stations or to and from the network server. Typically wireless local area networks (WLAN's) controlled by a central access point will provide better throughput performance.
The license-free national information structure (U-NII) radio spectrum bands in the United States are assigned to 5.15–5.25, 5.25–5.35, and 5.725–5.825 GHz, and are ideal for wireless ad-hoc LAN communication use. The IEEE-802.11a protocol prescribes using a training sequence comprising a preamble that enables a receiver to lock on to the carrier and helps get the data demodulation going.
The IEEE-802.11a burst transmission begins with a two-part preamble, e.g., a short preamble and a long-preamble. The exact boundary point between the short and long preambles is important to the receiver's subsequent demodulation process, and must be found quickly in an environment where the carrier frequency and code phase are uncertain. Signal fading, multipath interference, and channel distortion can make signal acquisition less certain in a typical receiver.